The adjustability of an antenna means in this description that a resonance frequency of the antenna can be changed electrically. The aim is that the operating band of the antenna around the resonance frequency always covers the frequency range, which the function presumes at each time. There are different causes for the need for adjustability. As portable radio devices, like mobile terminals, are becoming smaller thickness-wise, too, the distance between the radiating plane and the ground plane of an internal planar antenna unavoidably becomes shorter. This results in e.g. that the antenna bandwidths will decrease. Then, as a mobile terminal is intended for operating in a plurality of radio systems having frequency ranges relatively close to each other, it becomes more difficult or impossible to cover frequency ranges used by more than one radio system. Such a system pair is for instance GSM1800 and GSM1900 (Global System for Mobile telecommunications). Correspondingly, securing the function that conforms to specifications in both transmitting and receiving bands of a single system can become more difficult. If the system uses sub-band division, it is advantageous, from the point of view of the radio connection quality, if the resonance frequency of the antenna can be tuned in a sub-band being used at each time.
In the invention described here the antenna adjusting is implemented by a switch. The use of switches for the purpose in question is well known as such. For example the publication EP1 113 524 discloses an antenna, where a planar radiator can at a certain point be connected to the ground by a switch. When the switch is closed, the electric length of the radiator is decreased, in which case the antenna resonance frequency becomes higher and the operating band corresponding to the resonance frequency is displaced upwards. A capacitor can be in series with the switch to set the band displacement as large as desired. In this solution the adjusting possibilities are very limited.
In FIG. 1 there is a solution including a switch, known from the publication EP 1 544 943. Of the antenna base structure, only the radiator 120 is drawn in the figure, which radiator can be a part of a larger radiating plane. The antenna comprises, in addition to the base structure, an adjusting circuit with a parasitic element 131, a transmission line 132, a two-way switch 133, a first reactive circuit X1 and a second reactive circuit X2. The head end of the first conductor of the transmission line is connected to the parasitic element, and the head end of the second conductor is connected to the ground. In practice, the second conductor can belong to the ground plane, which as such has no head and tail end. Each reactive circuit includes for example two or three reactive components. The transmission line 132 will be terminated, depending on the switch state, by one of the reactive circuits. When the switch is controlled so that its state changes, the electric length and resonance frequency of a certain part of the antenna change. This means that the corresponding operating band is displaced.
The solution according to FIG. 1 is intended for a multi-band antenna. In it the influence of the adjusting can be directed, when needed, only on one operating band of the antenna, and a good impedance matching can be arranged for the antenna in the band to be displaced. These matters are due to that there are several variables when designing the adjusting circuit. However, the solution is suitable only for the antennas of PIFA type, and the parasitic element used in it increases the structure costs.